Integrated circuit assemblies generally include a chip, one face of which is provided with a layer of adhesive for affixing the integrated circuit or circuits. The chip may include an electrical mesh network of printed circuits and may cooperate with a cooling element, such as a dissipator glued onto the other face of the chip.
Increasingly large scale integration of integrated circuits on a silicon chip has the effect of increasing the density of the heat that has to be dissipated. Even if certain technologies for manufacturing transistors lessen their dissipation, the increase in the heat density to be dissipated remains, because of the very large scale integration that has by now been attained. To retain the advantage obtained by very large scale integration of integrated circuits, the chip assemblies, in order to be incorporated into a machine, must be adapted perfectly to all the characteristics of the chips. The solution that is adopted is often the best compromise between the mechanical, electrical and thermal characteristics of the elements involved. Since the mechanical and electrical characteristics are ordinarily known with precision, the best compromise cannot be obtained without precise measurement of the thermal characteristics.
Moreover, it is very interesting to measure the thermal resistance of all the elements comprising the heat dissipation route between a chip and the ambient air under actual operating conductions of a chip assembly. In these concrete instances, the measurements often diverge widely from the total resistance calculated on the basis of the thermal conductivities of the various materials used in the heat dissipation route. It is therefore very important to locate the source of these divergences in the heat dissipation route. For example, they may originate in the interfaces, in the porosity of a material that was made porous when it was applied, or from a drift in the thermal resistance of a material under the influence of the heat or some other external factor.
The Japanese patent published as No. 57-132046 describes a method and an apparatus for measuring the conductivity and thermal capacity of a specimen. This apparatus includes a copper plate that cooperates with a thermostat acting as a hot source, and a thin plate of acrylic. The two plates have two faces facing one another and intended for clamping the specimen, and each face is provided with a thermocouple. The method comprises, first, placing the specimen on the acrylic plate and waiting until it has uniformly assumed the ambient temperature. Then the copper plate is placed in the thermostat to make it assume a uniform temperature, and then is taken out of the thermostat and rapidly put into contact with the specimen. After 30 seconds, the temperatures at the thermocouples are read. Based on the temperature values ascertained and on the thickness of the specimen, in particular, the thermal conductivity of the specimen is determined, using charts prepared beforehand as a function of various parameters.
This method described in the above-identified Japanese Pat. No. 57-132046 is limited to measuring the thermal resistance of a specimen of a uniform thickness. Thus it cannot be used for measuring the thermal resistance of an assembly having a composite structure, the components of which have different surface areas and non-uniform thicknesses (in particular because they are not parallel). This is typical of an integrated circuit assembly. Furthermore, in the apparatus of the prior art the acrylic plate is a thermal insulator, which cannot act as a dissipator or as a cold source. As a result, the method of the prior art is based on the specific (or mass) heat that is transferred from the copper plate to the specimen per unit of time. In other words, there is no continuous circulation of a flow of heat between a hot source and a cold source. Thus it is not possible, with this method, to measure the thermal resistance of an integrated circuit assembly subject to actual operating conditions. This method is also poorly suited to specimens having a relatively large mass or strong thermal resistance. Moreover, the act of withdrawing the copper plate from a thermostat results in uncontrollable energy losses. The method of the prior art thus cannot handle wide divergences of temperature between the plates, such as those existing for example between the active components of the chip and its dissipator.
Finally, the reliance on charts presumes prior experience with known specimens subjected to particular, restrictive conditions. Also, calculation using intrapolation or extrapolation is involved. In conclusion, the method and apparatus of the prior art are unable to provide very precise and reliable measurement of the thermal resistance of any element of any kind, no matter whether its structure is simple or composite, and no matter what conditions it is subjected to.
Measuring the thermal resistance of a chip assembly subjected to actual operating conditions is accordingly done by a different method. The standard measuring method is performed only on the chip and its assembly. The chip comprises the hot source, and its assembly is connected to its cold source subjected to actual operating conditions. Then the thermal flux Q produced by the chip and transmitted to the cold source is measured, and hence the temperature T1 of the hot source and the temperature T2 of the cold source. The thermal resistance R between the chip and its cold source is calculated using the formula R=(T1-T2)/Q.
This method can provide only an overall value of the thermal resistance of the entire heat dissipation route between the chip and the ambient air. Accordingly, a defect along this route cannot be located with this method. Furthermore, this measurement proves to be increasingly inaccurate, the smaller the dimensions of the elements. Also, the precision of the measurement is affected by the imprecision of the measurement of the thermal flux Q effectively transmitted from the chip to the cold source. In fact, if the theoretical value of Q corresponds to the electric power furnished to the chip, in practice some of this power dissipates, by natural convection. As a consequence, the precise determination of Q in the above equation cannot be made unless the power losses q caused by natural convection are known.